Track URL of source repository in management repository

To be able to track the repository with the package sources that a particular pkgbase has been built from, we need to be able to track the URL of the source repository per pkgbase in the management repository.

This field is not written to the sync db (e.g. json2db).

The field should be able to be guarded with a configurable URL match (e.g. https://gitlab.archlinux.org/) to ensure, that no packages from outside sources end up in a target binary repository).